To protect environment, automobile engines have recently been provided with improved fuel efficiency, lower emission, and higher power, and improvements have been conducted particularly to reduce the friction of sliding members of pistons for higher fuel efficiency. The reduction of the tension of piston rings is particularly important, and because 50% or more of the tension of all piston rings is occupied by that of an oil control ring, measures have been conducted to reduce its tension. However, the tension reduction of an oil control ring likely reduces its followability to a cylinder liner wall, resulting in increased oil consumption. Further, in a three-piece oil control ring comprising a spacer expander having upwardly extending projections, which are called seating tabs, for pushing inner surfaces of side rails to generate tension, two upper and lower side rails are likely rotated separately in a circumferential direction, so that their gaps may overlap each other. In such a case, a lubricating oil remaining in the gaps is conveyed into a combustion chamber of an internal engine, resulting in too much supply of the lubricating oil and excessive consumption of an oil.
To prevent side rails from rotating separately from a spacer expander, for example, Japanese Utility Model Application Publication 59-99153 A discloses the formation of pluralities of grooves or recesses on surfaces of pads (seating tabs). The formation of pluralities of grooves or recesses provides the seating tabs with large surface roughness to prevent the rotation of side rails.
Like Japanese Utility Model Application Publication 59-99153 A, Japanese Utility Model Application Publication 60-14261 A discloses a method for preventing the relative rotation of side rails to a spacer expander by making the curvature of nail portions (seating tabs) equal to the curvature of an inner surface of each side rail, in addition to roughening the contact surfaces of seating tabs.
Japanese Utility Model Application Publication 1-78768 A discloses the formation of fine projections and recesses, for example, saw-like projections and recesses having a pitch of 25-250 μm and height of 15-180 μm, on seating tab surfaces, to increase friction resistance to prevent the rotation of side rails. As a method for working such seating tabs of a spacer expander, JP 3-193221 A discloses the use of an apparatus schematically shown in FIGS. 8(a) to 8(c), which presses tools having fine projections and recesses to side-rail-pushing surfaces of seating tabs, thereby transferring the projections and recesses of the tools to the side-rail-pushing surfaces.
Japanese Utility Model Application Publication 6-69522 A discloses the formation of projections of 0.006-0.060 mm on outer surfaces of seating tabs. In this case, only projections come into contact with side rails with large contact pressure, thereby preventing the relative rotation of side rails. It also describes that as a problem of Japanese Utility Model Application Publication 1-78768 A, decrease in an effective contact area accelerates the wear of inner surfaces of side rails, resulting in premature wearing of fine projections.
JP 2001-132840 A discloses the formation of projections as thick as 0.07-0.2 mm in center portions of seating tabs. Like Japanese Utility Model Application Publication 6-69522 A, this prior art utilizes the concept that the rotation of side rails is prevented by reducing contact areas, thereby increasing contact surface pressure. Because the projections are thick, such effects are kept for a long period of time.
JP 2003-148617 A discloses the formation of two or more grooves and three or more flat portions in a circumferential direction on seating tab surfaces, the total circumferential width of the flat portions being 30-70% of the circumferential width of seating tabs.
Any of the above conventional measures for preventing the separate rotation of side rails increases friction resistance on the contact surfaces of seating tabs of the spacer expander. To increase sliding resistance, projections are formed to adjust surface roughness, thereby increasing pushing pressure per a unit area. However, wearing occurs unexpectedly even when projections of sufficient height are formed, resulting in decreased friction resistance of contact surfaces, and thus the separate rotation of side rails. It has been found that the rotation of side rails is also affected by the swinging of a piston due to the deviation from roundness of a cylinder liner, clearance between a piston and a cylinder liner, and the shape of a piston, etc. An oil control ring having projections in seating tabs are subject to vibration, repeated or varying stress, etc., by this swinging phenomenon of a piston. As a result, it suffers so-called fretting fatigue, by which cracks propagate from damaged portions, resulting in destroying projections of sufficient height.